1. Field of the Invention
The present invention relates to a fixing device including a heater and a pressurizer so as to fix a recording sheet of paper passing through a nip region between the heater and the pressurizer, and relates to an image forming apparatus including the fixing device.
2. Discussion of the Background
Conventionally, in an electrophotographic image forming apparatus, a recording sheet of paper with a transferred unfixed toner image is conveyed to a fixing nip region between the heater heated by a heat source and a pressurizer pressed to the heater, so as to be heated and pressed, and hence the unfixed toner image is fixed onto the recording sheet of paper, as widely performed. As the fixing device for fixing toner onto the recording sheet of paper, there is a conventional one in which a fixing belt is heated by heating means such as a halogen heater or a ceramic heater, and the recording sheet of paper is pressed to the fixing belt by pressure.
In recent years, in order to reduce emission of ultrafine particles and power consumption, the fixing device is required to set the temperature to a limit value that can secure the fixing strength. For this reason, highly accurate control is necessary for adjusting temperature of the fixing device, and therefore a non-contact thermistor that does not damage the surface of the fixing belt is usually used as a temperature sensor for measuring surface temperature of the fixing belt.
There is proposed a fixing device for performing the above-mentioned temperature control, including the temperature sensor disposed at a part of the fixing belt where thermal deformation is apt to occur (see Japanese Unexamined Patent Application Publication 2013-164438). In addition, there is proposed a fixing device including the temperature sensor for temperature control, and displacement detection means for detecting deformation of a heating pipe to be a heating member via the fixing belt, so as to prevent breakage of a component of the fixing device (see Japanese Unexamined Patent Application Publication 2013-057896).
However, a non-contact temperature sensor has a large dependence on distance to the object to be measured. In a structure of locally heating the fixing belt, the thermal deformation of the fixing belt causes deterioration of measurement accuracy of the temperature sensor. In particular, when using a free belt type fixing belt, a distance between the fixing belt and the temperature sensor is apt to vary. As a result, surface temperature of the fixing belt may not be accurately detected, and hence abnormality of the temperature control may occur.
In the structure of Japanese Unexamined Patent Application Publication 2013-164438, the temperature sensor is disposed at a position that can reduce influence of thermal deformation of the fixing belt, but the dependence on distance to the object to be measured causes detection error of the temperature sensor. As a result, the fixing device required to perform high accuracy temperature control is largely affected by a small error, resulting in a malfunction such as insufficient fixation of toner on the recording sheet of paper.
In addition, in the structure of Japanese Unexamined Patent Application Publication 2013-057896, thermal deformation of the fixing belt or the heating member is detected by the displacement detection means of a contact type, but a detection result of the displacement detection means is not reflected on the temperature control based on a measurement result of the temperature sensor. For this reason, the structure cannot perform high accuracy temperature control though it can prevent breakage of a component of the fixing device due to thermal deformation. Further, because the displacement detection means is a contact type, when the displacement detection means become a non-contact state with the fixing belt, displacement thereof cannot be measured, and hence a correction process in the temperature control cannot be performed.